Room-temperature quantum noise limited spectrometry and methods of the same

ABSTRACT

In one embodiment, a heterodyne detection system for detecting light includes a first input aperture adapted for receiving first light from a scene input, a second input aperture adapted for receiving second light from a local oscillator input, a broadband local oscillator adapted for providing the second light to the second input aperture, a dispersive element adapted for dispersing the first light and the second light, and a final condensing lens coupled to an infrared detector. The final condensing lens is adapted for concentrating incident light from a primary condensing lens onto the infrared detector, and the infrared detector is a square-law detector capable of sensing the frequency difference between the first light and the second light. More systems and methods for detecting light are described according to other embodiments.

RELATED APPLICATIONS

The present application claims priority to a U.S. Provisional PatentApplication filed Mar. 30, 2010, under Appl. No. 61/319,130, which isincorporated herein by reference.

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

The present invention relates to spectrometry, and more particularly, toquantum noise limited performance in spectrometers.

BACKGROUND

Infrared spectral detection systems are useful in a variety ofapplications, especially for ground and space based persistentsurveillance. Impediments to field deployment of hyper-spectraldetection systems include the detector's overall size, weight, andenergy usage, which are driven in large part by the requirements of thecooling system for the spectrometer and detector. Therefore, roomtemperature operation of hyper-spectral detection systems has long beensought to eliminate the vast cooling requirements of conventionalhyper-spectral detection systems. However, intrinsic semiconductordetector noise from dark current and radiation from the spectrometerwalls presents a significant limitation on system signal-to-noiseperformance.

One conventional solution to this problem for high-spectral resolutionis the use of signal multiplexing, such as Fourier transformspectrometry. This approach detects the entire signal spectral bandsimultaneously, thereby increasing the competitive position of thesignal current with respect to the noise sources. However, even signalmultiplexing is overwhelmed by large background flux at elevatedtemperatures, and at high spectral resolution, this approach fails toachieve quantum noise limited (QNL) performance. QNL performance, wheredetection sensitivity is limited only by the quantum shot noise of thesignal, is a widely used measure with which to determine the performanceof signal detection in spectrometry applications.

Another approach uses thermal bolometer detectors for room temperatureoperation. However, Johnson-Nyquist noise imposes a requirement on thesesystems for broadband operation, with bandwidths in a range of about10-90 cm⁻¹. In addition, the thermal response times for bolometersimpose limits on the operational speed of a spectrometer.

Another approach has been used to achieve QNL performance by heterodynedetection by using a single frequency laser source. In heterodynedetection, a known light source is combined with an incoming signal on anon-linear detector to produce beat frequencies that are amplified anddetected. If the known light source generates more detector signal thanthe sources of noise, a QNL results. Heterodyne detection hashistorically depended on sufficiently bright light sources provided bynarrow beam lasers. These heterodyne approaches rely on tuning thenarrow band laser over the spectral region of interest. This tuning ofthe laser over the spectral band of interest can considerably reduce theefficiency in gathering spectral information over an extended spectralregion, however. Conventional incandescent light sources, whilepotentially bright enough, are extremely inefficient since a substantialfraction of their output is outside the spectral range of interest.

Another approach illustrated in the prior art is to use an immersioncondensing lens to reduce the size of the detector. Because dark currentis proportional to detector size, this discriminates against backgroundand dark current. A factor of 16 in focal area reduction has beenachieved by VIGO for HgCdTe detectors in the long-wave region throughthe use of high index immersion lens technology. However this reductionis not enough to overcome dark currents sufficiently to allow QNLperformance.

Therefore, all these approaches have failed to overcome the imposedlimitations on size, weight, and energy usage imposed by coolingrequirements. Even if QNL performance is achievable with hyper-spectraldetection systems at elevated temperatures, the size, weight, and energyusage limitations placed on implementation of these devices makes theiruse impractical. Therefore, a hyper-spectral detection system whichcould reach QNL performance while achieving practical implementationregarding size, weight, and energy usage would be very beneficial,particularly in military and law enforcement efforts to deploy highlysensitive instruments capable of ground-based persistent surveillanceand micro-power space surveillance.

SUMMARY

In one embodiment, a heterodyne detection system for detecting lightincludes a first input aperture adapted for receiving first light from ascene input, a second input aperture adapted for receiving second lightfrom a local oscillator input, a broadband local oscillator adapted forproviding the second light to the second input aperture, a dispersiveelement adapted for dispersing the first light and the second light, anda final condensing lens coupled to an infrared detector. The finalcondensing lens is adapted for concentrating incident light from aprimary condensing lens onto the infrared detector, and the infrareddetector is a square-law detector capable of sensing the frequencydifference between the first light and the second light.

In another embodiment, a method for detecting light includes receivingfirst light from a desired scene in a first input aperture, introducingsecond light produced by a broadband local oscillator to a second inputaperture, passing the first light from the desired scene and the secondlight produced by the broadband local oscillator through a dispersiveelement, concentrating the light from the dispersive element onto adetector pixel using a condensing lens coupled to the infrared detector,and detecting simultaneously the first light and the second light usingthe infrared detector.

Other aspects and embodiments of the present invention will becomeapparent from the following detailed description, which, when taken inconjunction with the drawings, illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a heterodyne spectrometer, according to oneembodiment.

FIG. 2A shows a plot of current (electrons/(pixel·sec)) versuswavelength (microns) expected from various sources, according to oneembodiment.

FIG. 2B shows a plot of current (electrons/(pixel·sec)) versuswavelength (microns) expected from various sources, according to oneembodiment using a condensing superlens.

FIG. 3 shows a schematic of a system including a condensing superlenscapable of condensing electromagnetic radiation in a cavity mode,according to one embodiment.

FIG. 4 shows a plot of streamlines for a cavity mode that propagates tothe smallest radii, according to one embodiment.

FIG. 5 shows a plot of the change in energy density from a mouth of acavity mode, according to one embodiment.

FIG. 6 is a flow diagram of a method for detecting light according toone embodiment.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

Unless otherwise specifically defined herein, all terms are to be giventheir broadest possible interpretation including meanings implied fromthe specification as well as meanings understood by those skilled in theart and/or as defined in dictionaries, treatises, etc.

It must also be noted that, as used in the specification and theappended claims, the singular forms “a,” “an” and “the” include pluralreferents unless otherwise specified.

In one general embodiment, a heterodyne detection system for detectinglight includes a first input aperture adapted for receiving first lightfrom a scene input, a second input aperture adapted for receiving secondlight from a local oscillator input, a broadband local oscillatoradapted for providing the second light to the second input aperture, adispersive element adapted for dispersing the first light and the secondlight, and a final condensing lens coupled to an infrared detector. Thefinal condensing lens is adapted for concentrating incident light from aprimary condensing lens onto the infrared detector, and the infrareddetector is a square-law detector capable of sensing the frequencydifference between the first light and the second light.

In another general embodiment, a method for detecting light includesreceiving first light from a desired scene in a first input aperture,introducing second light produced by a broadband local oscillator to asecond input aperture, passing the first light from the desired sceneand the second light produced by the broadband local oscillator througha dispersive element, concentrating the light from the dispersiveelement onto a detector pixel using a condensing lens coupled to theinfrared detector, and detecting simultaneously the first light and thesecond light using the infrared detector.

According to one embodiment, broadband infrared spectrometry based onthe principle of heterodyne detection that improves spectral lightgathering efficiency while maintaining sensitivity may be achieved. Toaccomplish this, light from a broadband local oscillator source, such asa broadband quantum cascade laser or photonic bandgap light source, maybe introduced into a dispersive spectrometer at an entrance aperture(window) that is physically or spatially displaced from an inputaperture (window) for receiving the desired scene light. The physicaldisplacement between signal and local oscillator translates, afterdispersion, into a physical displacement of their spectra on asquare-law detector array. Consequently, each pixel of the arrayresponds to two frequencies creating an intermediate beat frequency inthe detector output. In this way, hundreds of individual spectralchannels may simultaneously record a high resolution infrared spectrum,substantially improving the throughput of heterodyne spectrometry.Additionally, this approach allows for operation at a significantlyhigher detector temperature than conventional broadband spectrometers.

A broadband heterodyne approach with emerging bright light sources ispromising for facilitating low-noise, elevated-temperature infrareddetection. One advantage of using a heterodyne detection approach with abright light source sufficiently powerful for use in hyper-spectraldetection systems is evident from the signal-to-noise expression forheterodyne detection, presented below as Equation 1.S/N=(N _(s) ^(1/2) N _(LO) ^(1/2))/[(N _(s) +N _(LO) +N _(spec) +N_(dark) +N _(other))^(1/2)]  (Eq. 1)

In Equation 1, N_(s), N_(LO), N_(spec) N_(dark) are net signal, localoscillator, spectrometer wall, and dark current photoelectrons,respectively, as collected over the data integration time. Thisexpression exhibits achieving quantum noise limit (QNL) performance whenthe photoelectrons from the local oscillator exceed other noise sources.N_(LO) then dominates the denominator and cancels itself in thenumerator. The expression predicts that elevated-temperature QNLperformance is achievable with efficient and bright broadband lightsources. This is especially important in the infrared spectral region.

Now referring to FIG. 1, one embodiment of a spectrometer 100 utilizingheterodyne detection is shown, where an input window or aperture 102 forthe scene input 104 (source of light to be detected) may bespatially/physically displaced with respect to the input aperture 106for a bright local broadband oscillator 108. A dispersive element 110,such as a grating, may be used to disperse the signals from the sceneinput 104 and the bright local broadband oscillator 108 such that thesignals are dispersed according to wavelengths. This physicaldisplacement between signal and local oscillator translates, afterdispersion, into shifts in the wavelength in the focal plane of thespectrometer 100, e.g., a physical displacement of their spectra.Consequently, each pixel of the array responds to two frequenciescreating an intermediate beat frequency in the detector output. Auinfrared detector 112 registers the displacement shifts in thewavelength. The detector 112 may be an array of photodetectors as knownin the relevant art, according to one embodiment, and more preferably isa square-law detector capable of sensing a frequency difference betweenthe light from the aperture 102 and the light from the bright localbroadband oscillator 108. The photodetectors may be mixing elements of atype known in the art. Moreover, in various embodiments the infrareddetector may comprise one or more detector elements, such as a radiofrequency (RF) detector element, a quantum well photodetector element,etc. as would be understood by one having ordinary skill in the art uponreading the present descriptions. In referred embodiments the IRdetector comprises at least two detector elements. More preferably, atleast one of the detector elements is a RF detector element, at leastone other of the detector elements is a photodetector element, and theRF detector element is decoupled from the photodetector element. Inparticularly preferred embodiments, the RF detector element may includea carbon nanotube antenna The IR detector may be coupled (and preferablyclose-coupled) to a RF waveguide. The RF waveguide, in some approaches,may be configured spatially so as to be coplanar to the infraredphotodetector, and more particularly so that an upper surface of each ofthe RF waveguide and the IR detector lie substantially along a singleplane, or along substantially parallel planes. The RF waveguide mayfurther be configured to facilitate optical upconversion of RF energygenerated by the IR photodetector. The RF waveguide may, in additionalembodiments, be coupled to or include an electro-optic modulatorconfigured to interact RF energy with a co-linear laser beam to outputan optical electric field. The output optical electric field, in someapproaches, may be described by the relationshipE ₀=(E _(i)/2)exp[jω ₀ t){exp[jm ₁ cos(ω_(m) t)+exp[jm ₂(ω_(m) t)+jφ]},where

ω and ω₀ are the optical carrier frequency and modulation frequency,respectively.

φ is a constant phase difference between the two interferometer arms,and

m₁ and m₂ are the phase modulation amplitudes on each of the twointerferometer legs 1 and 2, respectively.

In one approach, where the two frequencies meet spatial and spectralcoherency requirements, they coincide at each detector pixel and combineto create a beat frequency with a bandwidth determined by the spectralresolution of the spectrometer 100.

According to various approaches, either, neither, or both apertures 102,104 may have a lens placed therein for affecting light from therespective input. The lens may be for light shaping, filtering,focusing, concentrating, condensing, etc., and may be of any type knownin the art.

In a preferred embodiment, the optical configuration meets an antennatheorem condition such that the product of the aperture area and thecollection solid angle is approximately equal to the square of the lightsource wavelength. Furthermore, spectral coherency is established by thebandwidth of the light intercepted by each pixel, ranging from about 3.0GHz for a 0.1 cm⁻¹ spectrometer to about 30 GHz for a 1.0 cm⁻¹spectrometer. Also, the physical displacement at the entrance maypreferably be at least twice the spectral bandwidth, in some approaches.

Recently, the development of photonic bandgap light sources andbroadband quantum cascade laser light sources makes it practical toconsider using broadband light sources in the spectrometer configurationshown in FIG. 1. Sources that emit light in an adjustable band centeredon the wavelength region of interest can be bright and efficient.Commercial photonic bandgap (PBG) sources are equivalent to 1000 Kblackbodies in bands of several hundred wavenumbers, from at least about500 cm⁻¹ to about 5000 cm⁻¹. Some virtues of the photonic crystalemitters include targeted spectral ranges and output efficiencies atabout 5%. The intensity of these sources is sufficient to outpacespectrometer wall flux at ambient temperature for operation atwavelengths shorter than 5 microns. This relaxes the requirement forcooling the entire spectrometer and requires only detector cooling. Theintensities of wall radiation and coherent PBG sources are presented inFIG. 2A. This shows that the PBG source has sufficient intensity toserve as a LO for a room temperature spectrometer in the mid-infraredregion.

The above analysis assumes that IR detectors have high frequencyoperation and sufficiently low dark current noise. Recent developmentsin band-structure engineered materials such as quantum well infraredphotodetectors (QWIPs) have the requisite operating frequencies, up to75 Ghz, but they have high dark currents that require operation atcryogenic temperatures. More recently, new options have opened up in“quantum engineering” of materials, particularly with the development ofquantum dot detectors, quantum dot IR photodetectors (QDIPs), and atomicclusters including “metallic semiconductors,” that are opening up wholenew vistas for searching for combinations of high speed and low darkcurrents. Limiting the location and density of states inhibits one ofthe major sources of dark current, Auger processes. Quantum clustersthat assign angular momentum to electron trajectories further restrictthe electron coupling options by conservation rules.

Developments along these lines are being pursued with Type II InAs/GaSbsuperlattice structures. The dark current of these detector types can besignificantly lower than that of QWIPs. One example is shown in FIG. 2Afor both 200K operation and 300K operation. While the PBG source isbright enough to operate the spectrometer at ambient temperature, abrighter source is needed to overcome detector current.

The development of broadband quantum cascade infrared lasers, offers apotential robust local oscillator option to implement this approach. A4μ to 5μ cw laser operating at 285K with 1 Watt output and 5% wall plugefficiency has been described. The 400 cm⁻¹ span provides about 2.5 mWatts per cm⁻¹ wave-number interval. This meets the local oscillatorrequirement by a wide margin over the detector dark current at ambienttemperature. Lowering the overall power consumption to 1 Watt yieldsabout 2×10¹⁵ photons/pixel-sec, more than 2000 times higher thandetector dark current. This allows for more than 200 spatial pixelscreating the opportunity for an imaging spectrometer. Adjusting the wellwidth of the QCL may provide a LO in the more important 3 to 4 micronspectral region. Comparable QCL sources in the long-wave infrared regionprovide sufficient power to allow room temperature spectrometeroperation. However, detector dark current in this spectral region iscurrently much too high to operate using uncooled detectors.

FIG. 2B illustrates the relative currents for signal, noise sources, andlocal oscillators expected with the addition of a plasmonic condensinglens (see below) that reduces the detector volume. This reductiondirectly translates into lower background radiation and lower detectordark current. Using a photonic bandgap broadband source as a localoscillator, a room temperature spectrometer coupled with a detectorcooled to 200K is possible. In the long-wave infrared, a roomtemperature spectrometer with a cryo-cooled detector is possible. In themid-infrared shorter than 5 microns, the combination of a QLC coupledwith a plasmonic condensing lens makes possible QNL operation with aroom temperature spectrometer and uncooled detector.

According to one exemplary embodiment, a spectrometer includes adetector, two entrance apertures, and a dispersive element. It may havea resolution of about 1 cm⁻¹ that may provide about a 30 GHz band passsystem. The two entrance apertures may have a spatial separation ofabout 30 microns that translates into an overlap of two colors separatedby about 30 GHz at the detector. This matches a frequency response offast quantum well detectors (about 32 GHz) that are available. Thesedetectors may be fabricated as single elements or arrays exceeding250×250 pixels and are multiquantum well structures of composition InAs(9)/GaAs (10) with a lattice period of 58 A. Custom designfilter/amplifier following circuits may be included, such as a singledetector element with external signal processing or an array withon-board pixel circuit elements. Of course, this is just one exemplaryembodiment, and many other approaches may be used in designingdimensions and materials of construction for a spectrometer according tovarious embodiments.

A spectrometer, according to any embodiment described herein, mayinclude diagnostic/performance measuring and analysis capability, eitheron-board or accessible through an interface with an external device. Toassess the performance of broadband spectrometer designs, a model may bedeveloped that includes key physics elements as comparative values.Inputs to the model may include spatial and temporal characteristics ofthe broadband source and the signal, filter function, spectraldispersion, spatial (frequency) offset of the local oscillator andsignal light, and general detector characteristics, such as efficiency,bandwidth, dark current, etc. The model may be embodied as computerreadable code, which may be capable of calculating the heterodyneefficiency due to matching of the local oscillator and signal fielddistributions at each pixel, the base band detector photocurrent in thepresence of Poisson shot noise, detector dark current, wall radiation,and other noise sources (as would be apparent to one of skill in therelevant art), and ultimately the signal-to-noise ratio as a function ofthe fundamental inputs. Performance may be assessed versus a filterspectral bandwidth and shape, as well as the heterodyne offset(intermediate frequency).

Additionally, a model may be produced to predict the improvementexpected from using immersion lenses to focus light onto each pixel. Theperformance model may leverage a time-domain heterodyne detection codefor vibrometry applications, to which may be added the finite localoscillator bandwidth associated with the filtered broadband source.

Now referring to FIG. 3, in another embodiment, heterodyne detection maybe further improved by placing a condensing superlens or immersionmicro-lens 302 in contact with detector pixels 304 such that thesuperlens 302 receives the scene input 310 and condenses the light,thereby increasing the signal-to-noise ratio (SNR) by a factor of n²,where n is the refractive index of the detector pixels 304 (n can be aslarge as four). In some embodiments, the superlens 302 may increase theSNR and allow QNL in the long-wave infrared range.

In yet another embodiment, a condensing superlens 302 may be used toreduce dark current (N_(dark) in Equation 1), which results fromdetection events in the complete absence of any light source.

In a preferred embodiment, the condensing superlens 302 may befabricated from silicon (Si) by reactive ion etching, but any formationtechnique may be used with any material suitable for transmission of aninput signal in a desired wavelength through a superlens. According toone embodiment, fields of Si cones 306 may be formed on SiO₂ and coatedwith highly reflective metal 308 as known in the art, such as gold,copper, silver, platinum, any combination thereof, etc. After coatingthe Si cone 306 with the reflective metal 308, properly formed cones 306may be selected for focused ion beam (FIB) modification in order topermit transmittance of light therethrough. In a preferred embodiment,the FIB modification may produce a superlens 302 with an aperturediameter approximately one-seventh ( 1/7) that of the wavelength oflight that is to be detected by the detector pixels 304. Furthermore,preferred embodiments may place the superlens 302 in direct contact withthe detector pixels 304 to improve light absorption therein.

Some exemplary dimensions are indicated in FIG. 3, with a length of thesuperlens being about 10 μm, a diameter at a receiving end being about3.5 μm, and a diameter at a transmitting end being about 0.47 μm. Ofcourse, any dimensions may be used, as would be apparent to one of skillin the art, particularly when taking into account a SNR increasedesired, a size of individual pixels in the photodetector, the materialused, the wavelength of light to be detected, etc.

General derivatives of the preferred embodiment of the condensingsuperlens 302 may be fabricated from any material known in the art, withthe only requirement being that the material be transparent to thewavelength of light to be detected. For example, IR light istransmittable through Si, which makes Si a possible material for asuperlens used to condense IR light for detection.

An electromagnetic wave can propagate in a conical cavity until thediameter of the cavity approaches one-half wavelength of light in thepropagating medium. The focal spot of a lens would be substantiallylarger, by twice the F-number, for a wave in a perfectly conductingcavity.

This discussion is based on treatment of cylindrical waveguides andfibers, which extends to a conical geometry. If the field cannot be freeof an axial component, the coefficient of this component satisfies awave-like equation, and from it the remainder of the field can bedetermined. The resulting solutions can be expressed analytically interms of Bessel functions. Electric and magnetic fields are assumed tohave the time dependence ε=Ee^(−iωt) and β=Be^(−iωt), respectively.

Maxwell's equations in a uniform nondissipative medium of permittivityε, permeability μ, wave number k=(με)^(1/2)ω are shown below in Equation8.∇×E=iωB ∇·E=0 ∇×B=(k ² /iω)E ∇·B=0  (Eq. 8)

However, because of the similarity of the E and B fields, Ψ mayrepresent either value, thus:∇×∇×Ψ=k ²Ψ  (Eq. 9a)∇·Ψ=0  (Eq. 9b)

In spherical coordinates (r, θ, φ) orthonormal unit vectors {e₁, e₂, e₃}are used in the {r, θ, φ} directions, for which Ψ=e₁Ψ₁+e₂Ψ₂+e₃Ψ₃, whichis referred to as {“radial,” “axial,” “circular,”}, respectively.Similarly, “transverse” means non-radial on the e₂, e₃ surface. Withs=sin(θ),∇·Ψ=(1/r ²)(r ²Ψ₁)_(,r)+(1/rs)(sΨ ₂)_(,θ)+(1/rs)Ψ_(3,φ)=0  (Eq. 10a)∇×Ψ=(e ₁ /rs)[(sΨ ₃)_(,0)−Ψ_(2,φ)]+(e ₂ /rs)[Ψ_(1,φ)−(rsΨ ₃)_(,r)]+(e ₃/r)[(rΨ ₂)_(,r)−Ψ_(1,0)]  (Eq. 10b)

Thus, according to Equation 10a which allows Ψ₂ and Ψ₃ to be replacedwith Ψ₁, the wave equation for Ψ₁ is:(1/r ²)[(r ²ψ₁)_(,rr)+(1/s)(sψ _(1,θ))_(,θ)+(1/s ²)ψ_(1,φφ) ]+k²ψ₁=0  (Eq. 11)Next a separable solution is sought to the wave Equation 11 having theform:ψ₁ ∝R ₁(r)Θ₁(θ)Φ₁(φ)  (Eq. 12)This leads to the following eigenvalue equations:(r ² R ₁)_(,rr)+(k ² r ² −l(l+1)R ₁=0  (Eq. 13c)(1/s)(sΘ _(1,θ))_(,θ)+(l(l+1)−m ² /s ²)Θ₁=0  (Eq. 13b)Φ_(1,φφ) +m ²Φ₁  (Eq. 13a)Substituting x=kr, X=x^(3/2)R₁, or R₁=x^(−3/2)X into Equation 13a yieldsBessel's equation:x ² X _(,xx) +xX _(,x)+(x ² −l(l+1)−¼)X=0  (Eq. 14)

However, the large value of l in present applications renders the ¼ termnegligible, and solutions are the Bessel and Hankel functions, and withυ=[l(l+1)]^(1/2), the functions have asymptotic forms as x approachesinfinity:

$\begin{matrix}{{{X_{\upsilon}(x)} = {{J_{\upsilon}(x)}->{\left( {{2/\pi}\; x} \right)^{1/2}{\cos\left( {x - {{1/2}\;\pi\;\upsilon} - {{1/4}\;\pi}} \right)}}}}\begin{matrix}{\upsilon^{2} = {l\left( {l + 1} \right)}} \\{= {{Y_{\upsilon}(x)}->{\left( {{2/\pi}\; x} \right)^{1/2}{\sin\left( {x - {{1/2}\;\pi\;\upsilon} - {{1/4}\;\pi}} \right)}}}} \\{= {{H_{\upsilon}^{(1)}(x)} = {{{J_{\upsilon}(x)} + {{\mathbb{i}}\;{Y_{\upsilon}(x)}}}->{\left( {{2/\pi}\; x} \right)^{1/2}{\mathbb{e}}^{{\mathbb{i}}{({x - {{1/2}\;\pi\;\upsilon} - {{1/4}\;\pi}})}}}}}} \\{= {{H_{\upsilon}^{(2)}(x)} = {{{J_{\upsilon}(x)} - {{\mathbb{i}}\;{Y_{\upsilon}(x)}}}->{\left( {{2/\pi}\; x} \right)^{1/2}{\mathbb{e}}^{- {{\mathbb{i}}{({x - {{1/2}\;\pi\;\upsilon} - {{1/4}\;\pi}})}}}}}}}\end{matrix}} & \left( {{{Eq}.\mspace{14mu} 15}a} \right)\end{matrix}$Wherein the functions are related by the expression:X _(υ) ′=X _(υ-1)−(υ/x)X _(υ)  (Eq. 15b)With the implicit e^(−iωt) time dependence, the Hankel functions H⁽¹⁾,H⁽²⁾, represent outgoing, ingoing traveling waves, respectively, and theBessel functions represent standing waves with equal components ofoutgoing and ingoing waves.

Since Y_(υ) and the Hankel functions diverge at small radii, they cannotbe physical solutions to the propagation into an empty loss-free cavitybecause “loss-free” implies perfect reflection. The Bessel function ofthe first kind, however, corresponds to a well-behaved standing wavewith perfect reflection; so, in a loss-free cavity that reaches asufficiently small diameter X_(υ)=J_(υ).

The solutions to Equation 13b are the Legendre polynomials (m=O) andassociated Legendre functions (m≠0)^(υ). The solutions to Equation 13care trigonometry functions.Θ₁ =P _(l) ^(m)(c) c=sin θ  (Eq. 16a)Φ₁={cos(mφ), sin(mφ)}  (Eq. 16b)Hence, ψ₁ is composed of:(ψ₁)_(lm)=ψ_(1lm) {x ^(−3/2) X _(υ)(x)}{P _(l) ^(m)(c)}{cos(mφ),sin(mφ)};  (Eq. 17)

-   -   ψ_(1lm) is a constant.        By analogy, in a waveguide, distinguish modes with transverse        electric “TE” fields (E₁=0) and transverse magnetic “TM” fields        (M₁=0). By this construction, Ψ₁≠0, so Ψ is the “non-transverse”        field, and ∇×Ψ is the “transverse” field.

This choice of modes allows construction of the transverse component ofΨ.(∇×Ψ)₁=(1/rs)[(sψ _(φ))_(,θ)−ψ_(θ,φ)]=0  (Eq. 18)Which will be satisfied for a scalar field ξ with:ψ₂=ξ_(,0.) ψ₃=(1/s)ξ_(,φ)  (Eq. 19)∇·Y=0 implies(1/s)(sξ _(,θ))_(,0)+1/s ²)ξ_(,φφ)=−(1/r)(r ²ψ₁)_(,r)  (Eq. 20)For a separable solution ξ=ξ_(lm)R_(ξ)(r)Θ,(θ)Φ_(ξ)(φ)ξ_(lm)=ψ_(1lm) /l(l+1)  (Eq. 21a)R _(ξ)=(x ² R ₁)_(,x) /x  (Eq. 21b)Θ_(ξ)=Θ₁, Φ_(ξ)=Φ₁  (Eq. 21c)Thus, the entire field for an eigenmode is(ψ₁)_(lm) =l(l+1)ξ_(/m) R _(ξ) {P _(l) ^(m)(c)}{cos(mφ), sin(mφ)}(ψ₂)_(lm)=ξ_(θ)−ξ_(lm) R _(ξ) {P _(l) ^(m)(c)_(,0)}{cos(mφ), sin(mφ)}(ψ₃)_(lm)=ξ_(,φ) /s=ξ _(lm) R _(ξ) {P _(l) ^(m)(c)/s}{−sin(mφ), cos(mφ)}(∇×Ψ)_(1lm)=0  (Eq. 22d)(∇×Ψ)_(2lm)=(1/rs)[ψ_(1,φ)−(rsψ ₃)_(,r)]=ξ_(lm) {k×R ₁ }{P _(l)^(m)(c)/s}{−sin(mφ), cos(mφ)}  (Eq. 22e)(∇×Ψ)_(3lm)=(1/r)[(rψ ₂)−ψ_(1,θ)]=ξ_(lm) {k×R ₁ }{P _(l)^(m)(c)_(,θ)}{−cos(mφ), −sin(mφ)}  (Eq. 22f)R _(ξ) =x ^(−3/2) X _(υ)(x)R _(ξ)=(x ² R ₁)_(,x) /x=x ^(−1/2) X _(υ-1)(x)−(υ−½)R ₁P _(l) ^(m)(c)_(,θ)=(l+m)(l−m+l)P _(l) ^(m−1)(c)−(mc/s)P _(l)^(m)(c)  (vi)The fields satisfy boundary conditions appropriate for a conductor atthe surface of the cone, θ=θ₀:E ₁(θ₀)=E ₃(θ₀)=0; B ₂(θ₀)=0  (Eq. 23)Which is satisfied whereP _(l) ^(m)(cos θ₀)=0(TM); P _(l) ^(m)(cos θ₀)_(,θ)=0(TE)  (Eq. 24)For a narrow cone with θ₀<<1, l will be large. In this case,P _(l) ^(m)(cos θ₀)=l ^(m) J _(m)(lθ)  (Eq. 25)However, (Equation 13b) may be approximated using s=sin θ≈θα² +P _(l) ^(m) _(,αα) αP _(l) ^(m) _(,θ)+(y ² −m ²)P _(l) ^(m)≈0 y=υθ υ² =l(l+1)  (Eq. 26)Which yields a more accurate approximation in the form of Bessel'sequation:P _(l) ^(m)(c)≈υ^(m) J _(m)(y)mP _(l) ^(m)(c)/s≈υ ^(m+1) mJ _(m)(y)/yP _(l) ^(m)(c)_(,θ)≈υ^(m=1) mJ _(m)′(y)=υ^(m+1) [J _(m−1)(y)−mJ_(m)(y)/y]  (Eq. 27a)In this small θ approximation, for TEE ₁=0E ₂ =E×R ₁ mJ _(m)(y)/y{−sin(mφ), cos(mφ)}E ₃ =E×R ₁ J _(m)′(y){−cos(mφ), −sin(mφ)}B ₁ =BυR ₁ J _(m)(y){cos(mφ), sin(mφ)}B ₂ =BR _(ξ) J _(m)′(y){cos(mφ), sin(mφ)}B ₃ =BR _(ξ) mJ _(m)(y)/y{−sin(mφ), cos(mφ)}R ₁ =x ^(−3/2) X _(υ)(x)R _(ξ)=(x ² R ₁)_(,x) /x=x ^(−1/2) X _(υ-1)(x)−(υ−½)R ₁B=υ ^(m+1)ξ_(lm) E=(iω/k)B  (Eq. 27b)For TM, B replaces E in Equation 27b, and the eigenvalue υ²=l(l+1)satisfies the boundary conditions at θ₀:υ_(nm) =y _(nm)θ₀  (Eq. 28)where y_(nm) is the n^(th) zero of J_(m)′ (TE) or J_(m)(TM). This simplerelation between υ and θ₀ motivates application of the small-anglelimit.

TABLE 1 The Zeroes of J_(m)’ and J_(m) y_(nm) (TE) y_(nm) (TB) n/m 0 1 2n/m 0 1 2 0 3.832 1.841 3.054 0 2.045 3.832 5.136 1 7.016 5.331 6.076 15.520 7.016 8.417 2 10.173 8.536 9.969 2 8.864 10.173 11.620

In one embodiment, the mode TE₁₁, for which υ=1.842/θ₀, and TM₁₀, forwhich υ=2.405/θ₀, since such modes propagate to the smallest radii.Where the cavity is driven by a linearly polarized external field, onlythe m=1 mode(s) will be excited. Streamlines for the lowest mode TE01will overlap with the external field and comprises the cavity mode thatpropagates to the smallest radii, as depicted in FIG. 4.

As an indicator of propagation into the cavity, use the energy densityin the electromagnetic field. The volume energy density is:u=½(ε|E| ² +|B| ²/μ)  (Eq. 29)Wherein the linear density may be expressed by:dU/dr=r ² ∫dθ sin θ∫dφu≈(x/kv)² ∫dyu∫dφu  (Eq. 30)From Equation 20, the density component for single modes may beevaluated as:dU/dR≈u*/(υk)²{2π:m=0, π:m≠0}I _(nm)(2/π)

_(υ)(x)  (Eq. 31a)u*=½ξE ²=½B ²/μ  (Eq. 31b)I _(nm)=∫^(Ynm) [J _(m)′²+(mJ _(m) /y)² ]ydy  (Eq. 31c)

_(υ)(x)=(π/2)[(x ² +fυ ²)|xR ₁|² +x ² |R _(ξ)|²]  (Eq. 31d)F=∫ ^(Ynm) J _(m) ² ydy/I _(nm)=1  (Eq. 31e)

Where energy density of a given mode at the mouth of cavity is known,the change in energy density from the mouth of the cavity is containedwithin

_(υ)(x). Thus for a standing wave X=J with perfect reflection from thecavity, the energy density function is as shown in FIG. 5.

In any of the embodiments described above, the broadband oscillator mayproduce a range of wavelengths (e.g., broadband quantum cascade laser),as opposed to a single-frequency laser. In preferred embodiments, thebroadband oscillator may further reduce energy usage by including theability to limit the range of frequencies produced that correspond to adesired detection range.

Now referring to FIG. 6, a method 600 for detecting light is shownaccording to one embodiment. The method 600 may be carried out in anydesired environment, including those shown in FIGS. 1 and 3, amongothers. Of course, the method 600 may include more or less operationsthan those shown in FIG. 6, as would be apparent to one of skill in theart upon reading the present descriptions.

In operation 602, first light is received from a desired scene in afirst input aperture.

In operation 604, second light produced by a broadband local oscillatoris introduced to a second input aperture. In one embodiment, the localbroadband oscillator may include a broadband quantum cascade laserhaving broad spectral bands between about 500 cm⁻¹ and about 5000 cm⁻¹.

In one embodiment, the second light from the broadband local oscillatormay be limited to a frequency that corresponds to a desired detectionrange, which may reduce power usage of the broadband local oscillator.

In one preferred approach, the first input aperture may be spatiallyoffset from the second input aperture. The spatial offset may be basedon a desired frequency and/or wavelength detection range, in someapproaches.

In operation 606, the first light from the desired scene and the secondlight produced by the broadband local oscillator is passed through adispersive element.

In operation 608, the light from the dispersive element is concentratedonto a detector pixel using a condensing lens coupled to the detector.In one embodiment, the condensing lens may include a plasmonic IRcondensing lens, as previously described.

lens makes possible QNL operation with a room temperature spectrometerand uncooled detector. In preferred approaches, the concentratingperformed in operation 608 may significantly reduce intrinsic detectornoise resulting from dark current, e.g. by a factor of at least 10 inone approach, a factor of at least 20 in preferred approaches, and afactor of about 60 in particularly preferred approaches. Thesereductions of intrinsic detector current resulting from dark currentthus represent an improvement over conventional immersion lensconfigurations, so much that the presently disclosed concentration inoperation 608 exhibits an approximately seven-fold reduction inintrinsic detector noise resulting from dark current when compared totypical immersion lenses.

According to some approaches, the plasmonic IR condensing lens may havea conical shape tapering inward toward the sensor, and may include a Sicore (or any other material as known in the art that is compatible withthe light being concentrated) having a conductive metal cladding onradial surfaces thereof. The conductive metal cladding may be anymaterial which operates to concentrate the light within the plasmoniclens, as known in the art, such as Au, Ag, Cu, etc. The conductive metalcladding may also be selected based on a gain provided by the conductivemetal cladding, in more approaches.

In one embodiment, an angle measured from perpendicular to a planarsurface of the sensor and a sidewall of the conical shape may be betweenabout 5° and about 12°, for example about 8.5°.

According to another embodiment, the conical shape may have a firstdiameter at a distal end adjacent the detector of less than aboutone-half wavelength of incident light in Si. Also, the conical shape mayhave a second diameter at an end receiving the incident light from thedispersive element of approximately a diffraction limit set by theprimary condensing lens. Of course, the dimensions may be chosen toincrease the likelihood of light being concentrated by the plasmoniclens, according to techniques that would be apparent to one of skill inthe art upon reading the present descriptions.

In operation 610, the first light and the second light aresimultaneously detected using the detector. In one approach, thedetector may include an array of photodetectors, as previouslydescribed.

In one preferred approach, the array of photodetectors may be sufficientin number to detect a desired frequency range with a desired resolution,in order to conserve energy usage of the array of photodetectors, amongother advantages.

According to one embodiment, a frequency of the first light and afrequency of the second light may coincide at each pixel of the detectorwith a bandwidth determined by a spectral resolution of a spectrometer.

In an optional operation, a coherent beat frequency may be created usingthe two frequencies, wherein each area of the first and second inputapertures times a collection solid angle of the spectrometer is about awavelength squared (about λ²).

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A heterodyne detection system for detectinglight, comprising: a first input aperture configured to receive firstlight from a scene input; a second input aperture configured to receivesecond light from a local oscillator input; a broadband local oscillatorconfigured to provide the second light to the second input aperture; adispersive element configured to disperse the first light and the secondlight; and a final condensing lens coupled to an infrared detector,wherein the final condensing lens is configured to concentrate incidentlight a primary condensing lens onto the infrared detector, and whereinthe infrared detector is a square-law detector configured to sense thefrequency difference between the first light and the second light,wherein the final condensing lens comprises a plasmonic infrared (IR)condensing lens having: a silicon (Si) core; a conductive metal claddingon radial surfaces of the Si core; and a conical shape tapering inwardtoward the sensor.
 2. The heterodyne detection system as recited inclaim 1, further comprising a radio frequency (RF) waveguide, whereinthe RF waveguide is configured to facilitate optical upconversion of RFenergy generated by the infrared photodetector.
 3. The heterodynedetection system as recited in claim 1, wherein at each detector pixel,a frequency of light from the scene input and a frequency of light fromthe oscillator input coincide with a bandwidth determined by a spectralresolution of the system, and wherein the detection system is configuredsuch that the two frequencies create a coherent beat frequency, whereineach area of the first and second input apertures times an inputcollection solid angle of the sensor is about a wavelength squared(about λ²).
 4. The heterodyne detection system as recited in claim 1,wherein the RF waveguide is coplanar to the infrared detector.
 5. Theheterodyne detection system as recited in claim 1, wherein the infraredphotodetector comprises: a photodetector element; and an RF detectorelement decoupled from the photodetector element.
 6. The heterodynedetection system as recited in claim 5, wherein the photodetectorelement comprises a quantum well infrared photodetector (QWIP), andwherein the RF detector element comprises a carbon nanotube antenna. 7.The heterodyne detection system as recited in claim 1, wherein thebroadband local oscillator comprises a broadband quantum cascade laserhaving broad spectral bands between about 500 cm⁻¹ and about 5000 cm⁻¹.8. The heterodyne detection system as recited in claim 1, wherein thefirst input aperture is spatially offset from the second input aperture,wherein the first and second light pass through the first and secondapertures in a same direction relative to each other.
 9. The heterodynedetection system as recited in claim 1, wherein the conductive metal ischosen from a group consisting of: Au, Ag, and Cu.
 10. The heterodynedetection system as recited in claim 1, wherein the conical shape has afirst diameter at an end adjacent the infrared detector of less thanabout one-half wavelength of the incident light in Si, and wherein theconical shape has a second diameter at an end receiving the incidentlight from the dispersive element of approximately a diffraction limitset by the primary condensing lens.
 11. A method for detecting light,the method comprising: receiving first light from a desired scene in afirst input aperture; introducing second light produced by a broadbandlocal oscillator to a second input aperture; passing the first lightfrom the desired scene and the second light produced by the broadbandlocal oscillator through a dispersive element; concentrating the lightfrom the dispersive element onto a detector pixel of an infrareddetector using a plasmonic infrared (IR) condensing lens coupled to theinfrared detector, the plasmonic IR condensing lens comprising: asilicon (Si) core; a conductive metal cladding on radial surfaces of theSi core; and detecting simultaneously the first light and the secondlight using the infrared detector, wherein the plasmonic IR condensinglens has a conical shape tapering inward toward the sensor, wherein theconcentrating reduces an intrinsic detector noise resulting from darkcurrent by a factor of at least
 20. 12. The method as recited in claim11, further comprising: generating radio-frequency (RF) energy using theIR detector in response to detecting the first light and the secondlight; and optically upconverting the generated RR energy.
 13. Themethod as recited in claim 12, further comprising creating a coherentbeat frequency using the two frequencies, wherein each area of the firstand second input apertures times a collection solid angle of thespectrometer is about a wavelength squared (about λ²).
 14. The method asrecited in claim 12, wherein the upconverting comprises: channeling thegenerated RF energy to a close-coupled RF waveguide; and interacting thechanneled RF energy with a laser beam, wherein the interacting generatesan optical electric field.
 15. The method as recited in claim 14,wherein the laser beam is co-linear to the channeled RF energy.
 16. Themethod as recited in claim 12, wherein the interacting is performed atan electro-optic modulator coupled to the RF waveguide.
 17. The methodas recited in claim 11, wherein the broadband local oscillator comprisesa broadband quantum cascade laser having broad spectral bands betweenabout 500 cm⁻¹ and about 5000 cm⁻¹.
 18. The method as recited in claim11, wherein the first input aperture is spatially offset from the secondinput aperture, and wherein the first and second light pass through thefirst and second apertures in a same direction relative to each other.19. The method as recited in claim 11, wherein the conductive metal ischosen from a group consisting of: Au, Ag, and Cu.
 20. The method asrecited in claim 11, wherein the conical shape has a first diameter at adistal end adjacent the detector of less than about one-half wavelengthof incident light in Si, and wherein the conical shape has a seconddiameter at an end receiving the incident light from the dispersiveelement of approximately a diffraction limit set by the primarycondensing lens.